Hey asikes, to give you some perspective, there is literally a thread compiling peoples challenges with Billie Jean because everyone is shocked by it.
Josh repeatedly says to accomplish the slow workouts because he continues to teach stuff like music theory and tips for string crossing, and you may find a medium or fast workout that you enjoy the rhythm to and are more apt to practice.
I came into this with some guitar playing so BJ was do-able with some practice, but if it’s just not happening I wouldn’t worry about it at all. If slow speeds are a struggle, try mapping out the notes ahead of time at your own speed. Memorizing the exercise pattern so you only focus on timing will definitely get you through the slow workouts.